My brita pitcher has a timer on it to let me know when it needs to be changed :) hahaha, yeah right. I work on a fishing boat and drink a gallon of water a day - I can tell by taste when it's time to change the filter, and it's much earlier than the timer says. In the winter, under normal use, the filters last considerably longer.
Though there are gradations and different kinds of activated carbon, yes what is in the brita is the same stuff that's in the filter.
Activated carbon uses the van der wals force to trap and hold stuff. AC has a physical maximum capacity, meaning that there are a finite number of attachment sites. Particles that have a high affinity for the carbon are drawn to an attachment site, where they are held. However, it is not a real chemical reaction. Once the attachment sites are filled, particles with a higher attraction for the carbon will bump out particles with less attraction. Those ejected particles will then bump out other particles that have an even lower affinity. So, the carbon begins to leach things back into the aquarium. However, what is leached back into the tank is exactly the same as what was initially removed. There is no change.
Soooo, how often one really needs to change the carbon is entirely dependent on their specific situation. They say every month, but in reality it can lose it's effectiveness much sooner than that. Or much later. If you never changed the water, the carbon would last longer than if you did lots of water changes.
However, the carbon losing it's effectiveness and leaching things back into the aquarium is NOT a big deal. Many people (myself included) do not use carbon at all, so the things the carbon adsorbs are never removed in the first place.....
So in conclusion, if you use carbon, it's best to just stick to a schedule. Just take a moment to think about your practices. If your tank is heavily stocked and receiving large, frequent water changes, you would want to change it more frequently than every month. If it's lightly stocked and you don't do a lot of water changes, then you could stretch it out to more than a month.